Multimode optical fiber transmission system including single mode fiber

ABSTRACT

Some embodiments of the disclosure relate to an optical transmission system that operates at a wavelength in the range from 950 nm to 1600 nm and that employs a single-mode optical transmitter and an optical receiver optically coupled to respective ends of a multimode fiber designed for 850 nm multimode operation. The optical transmission system also employs at least one single mode fiber situated within the optical pathway between the optical transmitter and the receiver and coupled to the multimode fiber.

RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.15/590,532, filed on May 9, 2017 which is a divisional application ofU.S. application Ser. No. 14/703,099, filed on May 4, 2015 which in turnclaims the benefit of priority to U.S. Provisional Application No.61/994,431, filed on May 16, 2014. The entire teachings of the aboveapplications are incorporated herein by reference.

FIELD

The present disclosure relates to optical transmission systems thatemploy multimode optical fiber, and in particular relates to the use ofat least one single mode fiber optically coupled to the multimodeoptical fiber.

BACKGROUND

No admission is made that any reference cited herein constitutes priorart. Applicant expressly reserves the right to challenge the accuracyand pertinence of any cited documents.

Optical fiber transmission systems are employed in data centers tooptically connect one optical device (e.g., a router, a server, aswitch, etc.) with another set of optical devices.

Current data centers are configured with multimode optical fiberscoupled to 850 nm multimode VCSELs (Vertical Cavity Surface EmittingLasers) light sources that provide modulated data signals to themultimode fibers. Such multimode fibers are used because the lightsources in the transceivers in the optical devices are multimode lightsources. Also, historically it has been easier to work with multimodefiber than single-mode fiber. Unfortunately, multimode fiber has asmaller bandwidth-distance product due to mode dispersion, which makesit difficult and expensive to extend the reach of the optical fibertransmission system while maintaining high-bandwidth transmission.Furthermore, utilizing a typical transmitter (that utilizes a 850 nmVCSEL) operating at 10 Gb/s as a source, current standard OM3 and OM4multimode optical fibers can transmit optical signal over a distance ofonly about 300 m to about 500 m, due to signal distortion caused by thechromatic dispersion introduced by silica material of these multimodefibers. As optical transmission speed moves to 25 Gb/s or higher, thisdistance becomes even shorter (75 m to 150 m) due to chromaticdispersion for the current standard OM3 and OM4 multimode optical fibersoperating at around 850 nm. Consequently, other ways of increasing thetransmission distance of the optical fiber transmission system withoutincurring the time, labor and expense having to replace the existingmultimode optical fiber are needed.

SUMMARY

Some embodiments of the disclosure relate to an optical transmissionsystem that operates at a wavelength in the range from 950 nm to 1600 nmand employs a single-mode optical transmitter and an optical receiveroptically coupled to respective ends of a multimode fiber designed for850 nm multimode operation. The optical transmission system also employsat least one single mode fiber situated within the optical pathwaybetween the optical transmitter and the receiver, and coupled to themultimode fiber.

One embodiment of the disclosure relates to an optical transmissionsystem that comprises:

a single-mode transmitter that generates modulated light having anoperating wavelength λo between 950 nm and 1600 nm;

an optical receiver configured to receive and detect the modulatedlight;

a multimode optical fiber that defines an optical pathway between thesingle-mode transmitter and the optical receiver, the multimode opticalfiber having a core with a diameter D₄₀ and a refractive index profileconfigured to optimally transmit light at a wavelength λ1 situatedbetween 840 nm and 860 nm and to propagate light in the LP01 mode at theoperating wavelength λo, the multimode fiber has a LP01 mode fielddiameter LP01MFD_(MMλ0) and a cutoff wavelength >1600 nm; and at leastone single mode fiber, at the operating wavelength λo; the at least onesingle mode fiber operably disposed in the optical pathway and situatedbetween the single-mode transmitter and the receiver, the at least onesingle mode fiber having cutoff wavelength λ_(SM)<1600 nm, and a lengthin the range from 1 cm to 20 m, and a core diameter D_(SM), and whereinD_(SM)<D₄₀, and

the mode field diameter MFD_(SM) of the single mode fiber at thewavelength λo is 0.7MFD_(SM)<LP01 MFD_(MM λ0)<1.3MFD_(SM). In someembodiments 0.8MFD_(SM)<LP01 MFD_(MM λ0)<1.2MFD_(SM). In someembodiments 0.9MFD_(SM)<LP01 MFD_(MM λ0)<1.1MFD_(SM). In some exemplaryembodiments the length of the single mode fiber is 5 cm to 20 m. In someembodiments 12 μm<MFD_(SM)<18 μm., and/or said single mode fibercomprises a core diameter D_(SM) of 15≤D_(SM)≤25 μm, and a relativerefractive core delta 0.8% to 0.25%.

According to some embodiments, the multimode fiber is multimoded at 980nm, 1060 nm, and/or 1310 nm and/or at 1550 nm, but propagates one ormore of these wavelengths in the LP01 mode. According to someembodiments, 12 μm<LP01 MFD_(MM λ0)<15 μm, and λo is between 950 nm and1080 nm (e.g., 980 nm or 1060 nm). According to some embodiments, 14μm<LP01 MFD_(MM λ0)<16 μm, and λo is between 1260 nm and 1340 nm.According to some embodiments, 14 μm<LP01 MFD_(MM λ0)<16 μm, and λo isbetween 1320 nm and 1340 nm. According to another embodiment, 14 μm<LP01MFD_(MM λo<)16 μm and λo is between 1540 nm and 1560 nm

According to one embodiment, 14 μm<LP01 MFD_(MM λ0)<16 μm and λo isbetween 1320 nm and 1340 nm. According to another embodiment, 14 μm<LP01MFD_(MM λ0)<16 μm and λo is between 1540 nm and 1560 nm. According toyet another embodiment 14 μm<LP01 MFD_(MM λ0)<16 μm, and 13μm<MFD_(SM)<19 μm.

According to some embodiments an optical transmission system comprises:

a multimode transmitter that generates modulated light having anoperating wavelength λ₁ situated between 840 nm and 860 nm;

an optical receiver configured to receive and detect the modulatedlight;

a multimode optical fiber that defines an optical pathway between themultimode transmitter and the optical receiver, the multimode opticalfiber having a core with a diameter D₄₀ and a refractive index profileconfigured to optimally transmit light at wavelength λ₁ situated between840 nm and 860 nm, and to propagate the LP01 optical mode at anotherwavelength λo, where λo>950 nm, the multimode fiber having a LP01 modefield diameter LP01MFD_(MMλ0) and 8.5 μm<LP01MFD_(MMλ0)<11 μm. Accordingto one exemplary embodiment, λo is between 1320 nm and 1360 nm.According to another exemplary embodiment λo is situated between 1540 nmand 1560 nm.

According to some embodiments an optical transmission system comprises:

a single mode transmitter that generates modulated light having anoperating wavelength λo between 950 nm and 1600 nm;

an optical receiver configured to receive and detect the modulatedlight;

a multimode optical fiber that defines an optical pathway between themultimode transmitter and the optical receiver, the multimode opticalfiber having a core with a diameter D₄₀ and a refractive index profileconfigured to optimally transmit light at a nominal wavelength atwavelength λ₁ situated between 840 nm and 860 nm and to propagate theLP01 mode at another wavelength λo, where

λo>950 nm, the multimode fiber has a LP01 mode field diameterLP01MFD_(MMλ0) and 8.5 μm<LP01MFD_(MMλ0)<11 μm; and

at least one single mode fiber operably disposed in the optical pathwayand situated between the single-mode transmitter and said multimodeoptical fiber and having a length in the range from 5 cm to 20 m,wherein the single mode fiber has a mode field diameter MFD_(SM) at theoperating wavelength λo, such that 0.7MFD_(SM)<LP01MFD_(MM λ0)<1.3MFD_(SM). In some embodiments 0.8MFD_(SM)<LP01MFD_(MM λ0)<1.2MFD_(SM). In some embodiments 0.9MFD_(SM)<LP01MFD_(MM λ0)<1.1MFD_(SM). According to one exemplary embodiment, λo isbetween 1320 nm and 1340 nm. According to another exemplary embodimentλo is situated between 1540 nm and 1560 nm. In some embodiments, 12μm<MFD_(SM)<18 μm., and/or said single mode fiber comprises a corediameter D_(SM) of 15≤D_(SM)≤25 μm, and a relative refractive core delta0.8% to 0.25%.

According to some embodiments the multimode fiber has a modal bandwidthof at least 2.5 GHz·Km at a wavelength λ₁.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding, and are incorporated into and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and operationof the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of one embodiment of optical fibertransmission system that employs a multimode transmitter and a singlereceiver optically connected by a multimode optical fiber 40;

FIG. 1B is a schematic diagram of one embodiment of optical fibertransmission system that employs a single-mode transmitter and asinglemode or receiver optically connected by a multimode optical fiber40;

FIG. 2A is a schematic diagram of one embodiment of optical fibertransmission system that employs a single-mode transmitter and amultimode receiver optically connected by a multimode optical fiber;

FIG. 2B is a schematic diagram of one embodiment of optical fibertransmission system that employs a single-mode transmitter and asingle-mode receiver 30S optically connected by a multimode opticalfiber;

FIGS. 3A and 3B are schematic diagrams of other example embodiments ofoptical transmission systems;

FIG. 4 illustrates MFD of the LP01 mode at 1310 nm wavelength of severalexemplary multimode optical fiber embodiments vs. fiber core radii;

FIG. 5 illustrates MFD of the LP01 mode at 1550 nm wavelength of severalexemplary multimode optical fiber embodiments vs. fiber core radii;

FIG. 6 shows bandwidth vs. wavelength for several exemplary MMFs;

FIG. 7 illustrates schematically a refractive index profile of oneexemplary MMF 40; and

FIG. 8 is a schematic diagram of one embodiment of optical fibertransmission system that employs a single mode transmitter opticallyconnected by a multimode optical fiber 40′, and SM fiber jumper(s)comprising SMF 50.

DETAILED DESCRIPTION

Additional features and advantages of the invention will be set forth inthe detailed description which follows and will be apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the following description together with theclaims and appended drawings.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and waveguide fiber radius.

The “relative refractive index” is defined as Δ=100×[n(r)²−n_(cl)²)/2n(r)², where n(r) is the refractive index at the radial distance rfrom the fiber's centerline, unless otherwise specified, and n_(cl) isthe average refractive index of the outer cladding region of thecladding at a wavelength of 850 nm, which can be calculated, forexample, by taking “N” index measurements (n_(C1), n_(C2), . . . n_(cN))in the outer annular region of the cladding, and calculating the averagerefractive index by:measurements (n_(C1), n_(C2), . . . n_(CN)) in the outer annular regionof the cladding, and calculating the average refractive index by:

$n_{C} = {\left( {1/N} \right){\sum\limits_{i = 1}^{i = N}\;{n_{Ci}.}}}$

In some exemplary embodiments, the outer cladding region comprisesessentially pure silica. As used herein, the relative refractive indexis represented by delta or A and its values are typically given in unitsof “%,” unless otherwise specified. In cases where the refractive indexof a region is less than that of the average refractive index of theouter cladding region, the relative index percent is negative and isreferred to as having a depressed index, and is calculated at the pointat which the relative index is most negative unless otherwise specified.In cases where the refractive index of a region is greater than therefractive index of average refractive index of the outer claddingregion, the relative index percent is positive and the region can besaid to be raised or to have a positive index, and is calculated at thepoint at which the relative index is most positive, unless otherwisespecified. With reference to core delta value, it is disclosed herein asmaximum % delta.

An “up-dopant” is herein considered to be a dopant which has apropensity to raise the refractive index relative to pure undoped SiO₂.A “down-dopant” is herein considered to be a dopant which has apropensity to lower the refractive index relative to pure undoped SiO₂.An up-dopant may be present in a region of an optical fiber having anegative relative refractive index when accompanied by one or more otherdopants which are not up-dopants. Likewise, one or more other dopantswhich are not up-dopants may be present in a region of an optical fiberhaving a positive relative refractive index. A down-dopant may bepresent in a region of an optical fiber having a positive relativerefractive index when accompanied by one or more other dopants which arenot down-dopants. Likewise, one or more other dopants which are notdown-dopants may be present in a region of an optical fiber having anegative relative refractive index.

Unless otherwise stated, the overfill (or overfilled (OFL)) bandwidth(BW) of an optical fiber is defined herein as measured using overfilledlaunch conditions at 850 nm according to IEC 60793-1-41 (TIA-FOTP-204),Measurement Methods and Test Procedures: Bandwidth. In the discussionbelow, bandwidth BW is understood to mean overfilled bandwidth unlessotherwise indicated.

The minimum calculated effective modal bandwidth (EBW) can be obtainedfrom measured differential mode delay spectra as specified by IEC60793-1-49 (TIA/EIA-455-220), Measurement Methods and Test Procedures:Differential Mode Delay.

The NA of an optical fiber means the numerical aperture as measuredusing the method set forth in IEC-60793-1-43 (TIA SP3-2839-URV2FOTP-177) titled “Measurement Methods and Test Procedures: NumericalAperture”.

The modeled bandwidth may be calculated according to the procedureoutlined in T. A. Lenahan, “Calculation of Modes in an Optical FiberUsing the Finite Element Method and EISPACK,” Bell Sys. Tech. J., vol.62, pp. 2663-2695 (1983), the entire disclosure of which is herebyincorporated herein by reference. Equation 47 of this reference is usedto calculate the modal delays; however note that the term dk_(clad)/dω²must be replaced with dk² _(clad)/dω², where k_(clad)=2π*n_(clad)/λ, andω=2π/λ, and n_(clad)=nc where is the average index of refraction of theouter cladding region. The modal delays are typically normalized perunit length and given in units of ns/km (or equivalently in units ofps/m). The calculated bandwidths also assume that the refractive indexprofile is ideal, with no perturbations such as a centerline dip, and asa result, represent the maximum bandwidth for a given design.

The term graded index, “α-profile” or “alpha profile,” as used herein,refers to a relative refractive index profile, expressed in terms of Δwhich is in units of “%”, where r is the radius and which follows theequation,

${{\Delta(r)} = {\Delta_{0}\left\lbrack {1 - \left( \frac{r}{R_{1}} \right)^{\alpha}} \right\rbrack}},$where Δ₀ is the relative refractive index extrapolated to r=0, R₁ is theradius of the core (i.e. the radius at which Δ(r) is zero), and a is anexponent which is a real number. For a step index profile, the alphavalue is greater than or equal to 10. For a graded index profile, thealpha value is less than 10. The term “parabolic,” as used herein,includes substantially parabolically shaped refractive index profileswhich may vary slightly from an a value of, for example, 2.0 at one ormore points in the core, as well as profiles with minor variationsand/or a centerline dip. The modeled refractive index profiles thatexemplify the invention have graded index cores which are perfect alphaprofiles. An actual fiber will typically have minor deviations from aperfect alpha profile, including features such as dips or spikes at thecenterline and/or a diffusion tail at the outer interface of the core.However accurate values of alpha and Δ₀ may still be obtained bynumerically fitting the measured relative refractive index profile to analpha profile over the radius range from 0.05 R₁≤r≤0.95 R₁. In idealgraded index fibers with no imperfections such as dips or spikes at thecenterline, Δ₀=Δ_(IMAX), where Δ_(IMAX) is the maximum refractive indexof the core. In other cases, the value from Δ₀ obtained from thenumerical fit from 0.05 R₁≤r≤0.95 R₁ may be greater or less thanΔ_(IMAX).

Additional features and advantages of the invention will be set forth inthe detailed description which follows and will be apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the following description together with theclaims and appended drawings.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and waveguide fiber radius.

The “relative refractive index” is defined as Δ=100×[n(r)²−n_(cl)²)/2n(r)², where n(r) is the refractive index at the radial distance rfrom the fiber's centerline, unless otherwise specified, and n_(cl) isthe average refractive index of the outer cladding region of thecladding at a wavelength of 850 nm, which can be calculated, forexample, by taking “N” index measurements (n_(C1), n_(C2), . . . n_(CN))in the outer annular region of the cladding, and calculating the averagerefractive index by:measurements (n_(C1), n_(C2), . . . n_(CN)) in the outer annular regionof the cladding, and calculating the average refractive index by:

$n_{C} = {\left( {1/N} \right){\sum\limits_{i = 1}^{i = N}\;{n_{Ci}.}}}$

In some exemplary embodiments, the outer cladding region comprisesessentially pure silica. As used herein, the relative refractive indexis represented by delta or A and its values are typically given in unitsof “%,” unless otherwise specified. In cases where the refractive indexof a region is less than that of the average refractive index of theouter cladding region, the relative index percent is negative and isreferred to as having a depressed index, and is calculated at the pointat which the relative index is most negative unless otherwise specified.In cases where the refractive index of a region is greater than therefractive index of average refractive index of the outer claddingregion, the relative index percent is positive and the region can besaid to be raised or to have a positive index, and is calculated at thepoint at which the relative index is most positive, unless otherwisespecified. With reference to core delta value, it is disclosed herein asmaximum % delta.

An “up-dopant” is herein considered to be a dopant which has apropensity to raise the refractive index relative to pure undoped SiO₂.A “down-dopant” is herein considered to be a dopant which has apropensity to lower the refractive index relative to pure undoped SiO₂.An up-dopant may be present in a region of an optical fiber having anegative relative refractive index when accompanied by one or more otherdopants which are not up-dopants. Likewise, one or more other dopantswhich are not up-dopants may be present in a region of an optical fiberhaving a positive relative refractive index. A down-dopant may bepresent in a region of an optical fiber having a positive relativerefractive index when accompanied by one or more other dopants which arenot down-dopants. Likewise, one or more other dopants which are notdown-dopants may be present in a region of an optical fiber having anegative relative refractive index.

Unless otherwise stated, the overfill (or overfilled (OFL)) bandwidth(BW) of an optical fiber is defined herein as measured using overfilledlaunch conditions at 850 nm according to IEC 60793-1-41 (TIA-FOTP-204),Measurement Methods and Test Procedures: Bandwidth. In the discussionbelow, bandwidth BW is understood to mean overfilled bandwidth unlessotherwise indicated.

The minimum calculated effective modal bandwidth (EBW) can be obtainedfrom measured differential mode delay spectra as specified by IEC60793-1-49 (TIA/EIA-455-220), Measurement Methods and Test Procedures:Differential Mode Delay.

The NA of an optical fiber means the numerical aperture as measuredusing the method set forth in IEC-60793-1-43 (TIA SP3-2839-URV2FOTP-177) titled “Measurement Methods and Test Procedures: NumericalAperture”.

The modeled bandwidth may be calculated according to the procedureoutlined in T. A. Lenahan, “Calculation of Modes in an Optical FiberUsing the Finite Element Method and EISPACK,” Bell Sys. Tech. J., vol.62, pp. 2663-2695 (1983), the entire disclosure of which is herebyincorporated herein by reference. Equation 47 of this reference is usedto calculate the modal delays; however note that the term dk_(clad)/dω²must be replaced with dk² _(clad)/dω², where k_(clad)=2π*n_(clad)/λ, andω=2π/λ, and n_(clad)=nc where is the average index of refraction of theouter cladding region. The modal delays are typically normalized perunit length and given in units of ns/km (or equivalently in units ofps/m). The calculated bandwidths also assume that the refractive indexprofile is ideal, with no perturbations such as a centerline dip, and asa result, represent the maximum bandwidth for a given design.

The term graded index, “α-profile” or “alpha profile,” as used herein,refers to a relative refractive index profile, expressed in terms of Δwhich is in units of “%”, where r is the radius and which follows theequation,

${{\Delta(r)} = {\Delta_{0}\left\lbrack {1 - \left( \frac{r}{R_{1}} \right)^{\alpha}} \right\rbrack}},$where Δ₀ is the relative refractive index extrapolated to r=0, R₁ is theradius of the core (i.e. the radius at which Δ(r) is zero), and a is anexponent which is a real number. For a step index profile, the alphavalue is greater than or equal to 10. For a graded index profile, thealpha value is less than 10. The term “parabolic,” as used herein,includes substantially parabolically shaped refractive index profileswhich may vary slightly from an a value of, for example, 2.0 at one ormore points in the core, as well as profiles with minor variationsand/or a centerline dip. The modeled refractive index profiles thatexemplify the invention have graded index cores which are perfect alphaprofiles. An actual fiber will typically have minor deviations from aperfect alpha profile, including features such as dips or spikes at thecenterline and/or a diffusion tail at the outer interface of the core.However accurate values of alpha and Δ₀ may still be obtained bynumerically fitting the measured relative refractive index profile to analpha profile over the radius range from 0.05 R₁≤r≤0.95 R₁. In idealgraded index fibers with no imperfections such as dips or spikes at thecenterline, Δ₀=Δ_(IMAX), where Δ_(IMAX) is the maximum refractive indexof the core. In other cases, the value from Δ₀ obtained from thenumerical fit from 0.05 R₁≤r≤0.95 R₁ may be greater or less thanΔ_(IMAX).

Reference is now made in detail to various embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same or like reference numbers andsymbols are used throughout the drawings to refer to the same or likeparts. The drawings are not necessarily to scale, and one skilled in theart will recognize where the drawings have been simplified to illustratethe key aspects of the disclosure.

The claims as set forth below are incorporated into and constitute partof this Detailed Description.

Various embodiments will be further clarified by the following examples.

At least one embodiment of the disclosure relates to an opticaltransmission system 10, 10′ that includes a multimode fiber (MMF) 40,40′. The multimode fiber 40, 40′ can operate both at a signal wavelengthλ₁ situated in a 840 nm-860 nm wavelength range (e.g., 845 nm<λ₁<855 nmrange, 850 nm) for multimode (MM) transmission, and at a longerwavelength λ₀ (for example, 980 nm, 1060 nm, 1310 nm, or 1550 nm) foressentially a single mode (SM) transmission. It is desirable for theoptical transmission systems 10 to have an operating wavelength λ₀longer than 950 nm (e.g., 980 nm, 1060 nm, 1310 nm or 1550 nm), in orderto lower chromatic dispersion due to the silica material of the opticalfiber. Thus, because the multimode fibers 40, 40′ in the embodiments ofthe optical transmission systems disclosed herein are capable ofoperating at both 850 nm for multimode transmission and at a longerwavelength λ₀ (i.e., λ₀>λ₁, where λ₀−λ₁>100 nm) for single modetransmission, they can be used with a commonly utilized 850 nm VCSELs(Vertical Cavity Surface Emitting Lasers), and at a later time theoptical transmission system can be advantageously upgraded by replacing850 nm VCSEL with a longer wavelength (e.g., λ₀>950 nm) light source,without replacing the multimode fiber(s) that is(are) already laid down.The longer wavelength light source can be, for example, 980 nm, 1060 nm,1310 nm or 1550 VCSELs, or a silicon photonics laser source operating ateither 1310 nm or 1550 nm, or a DFB (distributed feed-back) laseroperating at 950 nm to 1600 nm wavelength.

For example, in some embodiments of the optical transmission system 10,the longer wavelength light source that provides optical signals at thewavelength λ₀>950 nm is optically coupled to a relatively short length(e.g., 0.01 m to 20 m) of a single mode fiber (SMF) 50, 50′. Forexample, the relatively short length of SMF 50, 50′ may be in the formof a 0.01 m to 0.2 m SMF fiber stub type connector, or 0.5 m to 2 m inSMF jumper. The single mode fiber (SMF) 50, 50′ in turn can be directlycoupled to the multimode fiber 40, 40′ described herein. The longerwavelength light source and the SMF 50, could be provided, for example,in a single module, to be easily coupled to the MMF 40′, 40. Theupgraded optical transmission system 10 of these embodiments utilizes atleast one multimode fiber MMF 40, 40′ optimized for multimodetransmission in 840 to 860 nm wavelength range (for example at λ₁=850nm) and at least one single mode fiber SMF 50′, 50 capable of SMtransmission at a wavelength λ₀>950 nm, coupled to the multimodefiber(s) 40, 40′. The multimode fiber 40, 40′ is structured to propagatelight at the wavelength λ₀ in the LP01 mode and to have the mode fielddiameter of LP01 optical mode approximately equal (±30%, more preferably±20%) to the mode field diameter of the SM fiber 50, 50′. The SM fiber50, 50′ is optically coupled to the transiver 20, 30. The coupling lossfrom the LP01 mode of SMF to the LP01 mode of the MMF depends on themode field diameters (MFD). The coupling loss CL due to MFD mismatch canbe calculated using

${CL} = {{- 10}{\log\left\lbrack \frac{4}{\left( {{{MFD}_{SM}/{MFD}_{MM}} + {{MFD}_{MM}/{MFD}_{sM}}} \right)^{2}} \right\rbrack}}$The mode field diameter mismatch of not greater than ±30% helps to keepthe coupling loss not greater than 0.5 dB due MFD mismatch. For example,the SMF 50′, 50 may be situated between the transmitter 20 (containing alight source operating at a wavelength longer than 950 nm) and the MMF40, 40′. However, it may also be situated between the receiver 30 andthe MMF 40, 40′. In some embodiments of the optical system 10 the MMF40, 40′ is 100 m to 1000 m long.

In some exemplary embodiments the single mode fiber 50, 50′ is singlemoded at 1310 nm, and the multimode fiber 40, 40′ is structured to havemode field diameter (MFD) such that the LP01 mode propagating throughthe multimode fiber at 1310 nm is approximately equal to the MFD of thesingle mode fiber 50, 50′ at this wavelength (i.e., ±30%, or0.7MFD_(SM)<LP01 MFD_(MM)<1.3MFD_(SM) at λ₀=1310 nm). In someembodiments 0.8MFD_(SM)<LP01 MFD_(MM λ0)<1.2MFD_(SM), and in someembodiments 0.9MFD_(SM)<LP01 MFD_(MM λ0)<1.1MFD_(SM) at λ₀=1310 nm.

Also, for example, in some embodiments the single mode fiber 50, 50′ isa single mode fiber at 1060 nm, and the multimode fiber 40, 40′ isstructured to have mode field diameter (MFD) such that the LP01 modepropagating through the multimode fiber at λ₀ of about 1060 nm isapproximately equal to that of the single mode fiber 50, 50′ (i.e.,±30%, or 0.7MFD_(SM)<LP01 MFD_(MM)<1.3MFD_(SM) at λ₀). In someembodiments 0.8MFD_(SM)<LP01 MFD_(MM λ0)<1.2MFD_(SM), and in someembodiments 0.9MFD_(SM)<LP01 MFD_(MM λ0)<1.1MFD_(SM) at λ₀=1060 nm. Insome embodiments of the SMF 50, 12 μm<MFD_(SM)<18 μm, and/or the singlemode fiber 50 comprises a core diameter D_(SM) of 15≤D_(SM)≤25 μm, and arelative refractive core delta 0.8% to 0.25%.

Also, for example, in some embodiments the single mode fiber 50, 50′ isa single mode fiber at λ₀=1550 nm, and the multimode fiber 40, 40′ isstructured to have mode field diameter (MFD) such that the LP01 modepropagating through the multimode fiber at λ₀=1550 nm is approximatelyequal to that of the single mode fiber 50, 50′ (i.e., ±30%, or0.7MFD_(SM)<LP01 MFD_(MM)<1.2MFD_(SM) at λ₀=1550 nm). In someembodiments 0.8MFD_(SM)<LP01 MFD_(MM λ0)<1.2MFD_(SM), and in someembodiments 0.9MFD_(SM)<LP01 MFD_(MM λ0)<1.1MFD_(SM) at λ₀=1550 nm.

Also, for example, in some embodiments the optical fiber 50, 50′ ismultimoded at a wavelength λ₁ and propagates light in the LP01 mode at980 nm, or 1060 nm, or 1310 nm, or 1550 nm wavelength, or anotherwavelength λ₀ where λ₀−λ₁>100 nm, and the multimode fiber is structuredto have a mode field diameter such that the LP01 optical modepropagating through the multimode fiber 40, 40′ at this wavelength isapproximately equal (±30%, more preferably 20%, and even more preferably10%) to that of MFD of the single mode fiber 50, 50′ at that wavelength,to minimize coupling loses between the MMF and the SMF. Thus, accordingto these embodiments a multimode fiber 40, 40′ can be used in theoptical transmission system 10 for both transmission of signals providedby the 850 nm VCSEL light source(s), and for the single modetransmission of signal light provided to it from the single mode fiber,and the optical transmission system 10 advantageously does not requirecoupling devices utilizing mode converting lenses between the singlemode fiber and the multimode fiber. For example, the SMF and the MMF canbe advantageously spliced to one another, or butt coupled to oneanother, without the needing to have an intervening lens elementtherebetween.

According to some embodiments multimode fiber 40, 40′ can be used in theoptical transmission system 10 for both transmission of signals providedby the VCSEL light source(s) at the wavelength λ₁ (for example at λ₁=850nm), as well as for the single mode (LP01 mode at the wavelength λ₀)transmission to the single mode fiber 50, 50′ wherein the single modefiber 50, 50′ is situated between the MM fiber and the receiver. Inthese embodiments λ₀−λ₁>100 nm. In this embodiment, for example, themultimode fiber and the single mode fiber may be in physical contactwith one another, or may be coupled with an index matching fluid oradhesive therebetween, or may be separated by a small air gap d (e.g.,d<1 mm). The optical fiber(s) 50, 50′, 40, 40′ are structured such that0.7MFD_(SM)<LP01 MFD_(MM)<1.3MFD_(SM) at λ₀. Hence in this embodimentthe single mode fiber 50, 50′ strips the higher order optical modesbefore they propagate further into the optical system 10 (while allowingthe light in LP01 mode to propagate through). In these embodiments,advantageously, the optical transmission system 10 does not requirecoupling devices utilizing mode converting/matching lenses situatedbetween the single mode fiber 50, 50′ and the multimode fiber 40, 40′.

Some embodiments of the disclosure relate to an optical transmissionsystem 10 that operates at a wavelength in the range from 950 nm to 1600nm and that employs a single-mode optical transmitter and an opticalreceiver optically coupled to respective ends of a multimode fiberdesigned for 850 nm multimode operation. The optical transmission system10 employs at least one single mode fiber 50, 50′ within the opticalpathway between the optical transmitter and the receiver 20 and 30. Inthese embodiments the single mode fiber 50, 50′ ensures that only lightfrom LP01 mode at the wavelength is transmitted through the system,thereby advantageously enabling a system bandwidth of greater than 10GHz·km. The single mode fiber 50, 50′ can have a relatively short lengthL, e.g., 1 cm to 5 m, or 50 cm to 5 m. In some embodiments of the SMF50, 12 μm<MFD_(SM)<18 μm, and/or the single mode fiber 50 comprises acore diameter D_(SM) of 15≤D_(SM)≤25 μm, and a relative refractive coredelta 0.8% to 0.25%.

According to some exemplary embodiments, the physical core diameterD_(SM) of the single mode fiber 50′ is from 8.0 μm to 9.5 μm and thisfiber is coupled to the multimode fiber 40. In this embodiment themultimode fiber 40 has a relatively small core diameter D₄₀, forexample, 14 μm to 30 μm (and in some embodiments 15 μm≤D₄₀≤23 μm) whichis smaller than the 50 μm or the 62.5 μm diameters of conventional MMFused in transmission systems.

According to other embodiments the physical core diameter D_(SM) ofsingle mode fiber 50 is larger than that of the conventional SMF and hasa lower core delta (e.g. 0.1% to 0.25%) than that of the conventionalSMF. For example physical core diameter D_(SM) of single mode fiber 50is 14 μm to 24 μm and this SMF 50 can be coupled to the multimode fiber40′. The multimode fiber 40′ of these embodiments has a core diameterD₄₀, for example of 50 μm or 62.5 μm.

The single mode fiber 50, 50′ can be integrated within the optical pathin any of the components that define the optical path. For example, thesingle mode fiber 50, 50′ can be coupled to the transmitter 20 and/orthe receiver 30. The single mode fiber 50, 50′ can be spliced at eitheror both ends of the multimode fiber 40, 40′, for example to form part ofthe optical fiber link. In some examples, the upgrated opticaltransmission system 10 supports a data rate of greater than 10 Gb/s,e.g., 16 Gb/s, 25 Gb/s or even higher.

As shown in FIG. 1A, according to some embodiments the optical systemutilizes a multimode fiber (MMF) 40 that is suitable for both 850 nmmultimode transmission, and LP01 mode transmission at a longerwavelength λ₀ (e.g., 980 nm, 1060 nm, 1310 nm or 1550. The MMF 40 ofthis embodiment is designed for high bandwidth (BW) at a wavelength λ₁situated in 845 to 855 nm range (e.g., λ₁=850 nm). The fundamental mode(LP01) of MMF 40 has a mode field diameter (LP01 MFD_(MM)) that isapproximately equal to that of a standard single mode fiber 50′ such asSMF-28®, for example about 8.7-9.7 μm at 1310 nm, and about 9.8-10.8 μmat 1550 nm, and the MMF 40 preferably has a physical core diameter D₄₀of about 13-30 μm (e.g., 15 μm≤D₄₀≤23 μm). When the MMF 40 is used fortransmission in the optical transmission system 10′ at 850 nm shown inFIG. 1A, the MM transmitter is coupled directly to the MMF. At thereceiving end, the MMF 40 is coupled to a MM receiver.

When the MMF 40 of FIG. 1A is used for single mode transmission at alonger wavelength (λ_(0>950) nm, for example 1060 nm, 1310 nm or 1550nm) as shown in FIG. 1B, the SM transmitter may be coupled to a standardSMF 50′ that is coupled to the MMF 40 (with center alignment). Becausethe MFD of the fundamental mode of the MMF 40 is approximately the sameas the MFD of the standard SMF 50′, light provided from SM source 20S(or from the SMF 50′) to the MMF 40 is coupled into the fundamental modeLP01. At the receiving end, either a SM or a MM receiver can be coupleddirectly to the MMF 40, if no significant mode coupling loss occurs inthe MMF. However, if mode coupling happens during propagation in the MMF40, a standard SMF 50′ can be placed as a filter between the MMF and thereceiver, to strip the higher order modes.

FIG. 2A is a schematic diagram of an optical fiber transmission system(“system”) 10 that employs a single-mode (SM) transmitter 20S and amultimode (MM) receiver 30M optically connected by a multimode opticalfiber (MMF) 40 having a refractive index profile designed to optimallyoperate at a nominal wavelength of about 850 nm (i.e., has a “peakwavelength” in the 845 nm-855 nm range where mode dispersion isminimum). Because the MM optical fiber 40 described herein transmitsoptical signals at λ₀ wavelength in LP01 mode, the light launched fromthe SM transmitter 20S will propagate through the optical fiber 40, asif it was a single mode fiber.

FIG. 2B is similar to FIG. 2A but employs a SM receiver 30S. The SMtransmitter 20S can be one that is used in an optical communicationstransceiver, such as an LR, or LRM transceiver. The MM receiver 30M canbe one that is used in VCSEL-based transceivers or it can be a speciallydesigned MM receiver. SM transmitter 20S emits modulated light 22, whichin the example has a nominal wavelength λ₀ of at least 950 nm (e.g., 980nm, 1060 nm, 1200 nm, 1310 nm, or 1550 nm). More generally, SMtransmitter 20S emits light having a wavelength in the range from 950 nmto 1600 nm, and the systems and methods disclosed herein can haveoperating wavelengths in this range. In both embodiments of the opticaltransmission system 10 shown in FIGS. 2A and 2B, the SM fiber (notshown) can be coupled to the transiver 20, 30 and the multimode fiber,such that the MFD diameter of the SMF is approximately equal to that ofthe MMF, i.e., 0.7MFD_(SM)<LP01 MFD_(MM)<1.3MFD_(SM) at wavelength λ₀.Preferably, 0.8MFD_(SM)<LP01 MFD_(MM)<1.2MFD_(SM) at wavelength λ₀. Insome embodiments 0.9MFD_(SM)<LP01 MFD_(MM)<1.1MFD_(SM) at wavelength λ₀.

One embodiment of the optical system 10 is similar to that shown in FIG.1B but instead of MMF 40 the optical system 10 includes an existing or“legacy” 850 nm MMF 40′, such as existing OM2, OM3 or OM4 MM fiber withLP01 MFD in the range of 12-16 μm at wavelengths 950 nm to 1600 nm, withSM transceivers 20S operating at a wavelength λ₀ in the range from 950nm to 1600 nm (and in particular at about 1060 nm (i.e., 1060 nm±10 nm),or at about 1310 nm (i.e., 1310 nm±10 nm) or at about 1510 nm (i.e.,1510 nm±10 nm)) to transmit data within or between data centers overdistances of 100 m to 1000 m with possible data rates of 10 Gb/s orhigher (e.g., 25 Gb/s or higher, depending the system capability aslimited by power budget and bandwidth of the MMF 40′). In thisembodiment the SMF 50 is designed to be utilized with existing or“legacy” 850 nm MMF 40′, such as existing OM2, OM3 or OM4 MM fiber. Inthis embodiment the MMF 40′ is directly coupled to the SM fiber 50 thatis structured to have a MFD diameter (MFD_(SM)) at the wavelength λ₀such that 0.7MFD_(SM)<LP01 MFD_(MM)<1.3MFD_(SM). In some embodiments0.8MFD_(SM)<LP01 MFD_(MM)<1.2MFD_(SM) for example that 0.9MFD_(SM)<LP01MFD_(MM)<1.1MFD_(SM). The SMF 50 has a MFD in the 12-16 micron range (atwavelength λ₀ situated between 950 nm and 1600 nm), which is larger thanthe MFD of a standard SMF 50′ (e.g., larger than the MFD of SMF-28®) atthis wavelength. In some embodiments the core diameter (D_(SM)) of theSM fiber 50 that coupled to the existing OM2, OM3 or OM4 MM fiber 40′with the MFD of LP01 mode of about 12-16 μm at wavelength λ₀ situated in950 nm and 1600 nm range is, for example, 15 to 23 μm.

Thus, in some embodiments embodiment the optical system 10 includes MMF40′, such as existing OM2, OM3, or OM4 MM fiber with 12-16 μm MFD at thewavelength λ₀, with SM transceivers 20S operating at a wavelength λ₀ inthe range from 950 nm to 1600 nm (and in particular at about 980 nm (±10nm), 1060 nm (±10 nm), 1310 nm (±10 nm) or 1510 nm(±10 nm)) to transmitdata within or between data centers over distances of 100 m to 1000 mwith possible data rates of 10 Gb/s or higher (e.g. 25 Gb/s or higher,depending the system capability as limited by power budget and bandwidthof the MMF 40′). In these embodiment the MMF 40′ is directly coupled tothe conventional SMF fiber 50 and the SMF 50 is structured to have a MFDdiameter (MFD_(SM)) at the wavelength λ₀ such that 0.7MFD_(SM)<LP01MFD_(MM)<1.3MFD_(SM).

Note that in these embodiments the SM transmitter 30S discussed here canbe one that is designed based on an existing standard to work withsingle mode fiber (SMF). Such a SM transmitter 30S can be modified foruse with MMF to ensure better logistic management or compatibility withan existing installation. Note also that MMF 40′ is designed for optimaloperation at 850 nm but that the optical transmission system 10 operatesat a nominal wavelength in the range from 950 nm to 1600 nm, for exampleat a nominal wavelength of about 980 nm, 1060 nm, 1310 nm, or 1550 nm.

FIGS. 3A and 3B are schematic diagrams of example optical transmissionsystems 100 that are modified versions of systems 10 from FIGS. 2A and2B, and are configured to reduce the detrimental effects produced byhigher order modes, which have dramatically different group delaycompared to that of the fundamental LP01 mode. With reference to FIG.3A, system 10 includes either a single-mode or multimode receiver(“receiver”) 30 and a single fiber 50, 50′ arranged between MMF 40, 40′and receiver 30. In these embodiments the MMF 50 is coupled to the SMF40′, or MMF 50′ is coupled to the SMF 40. FIG. 3B is similar to FIG. 3Aand also includes a second single mode fiber 50, 50′ between SMtransmitter 20S and MMF 40. The two close-up insets of FIG. 3A showcross-sectional views of single mode fiber 50, 50′ and MMF 40, 40′.Single mode fiber 50, 50′ has a central core 52 surrounded by a cladding54. The central core has a diameter D_(SM). Single mode fiber 50, 50′preferably has a length in the range from 5 mm to 10 m. Multimode fiber40, 40′ has a core 42 of diameter D₄₀ surrounded by a cladding 44.

The core diameter D_(SM) of the single mode fiber 50, 50′ is smallerthan the core diameter D₄₀ of MMF 40, 40′. The smaller core diameterD_(SM) of the single mode fiber 50, 50′ acts to filter out higher-ordermodes that can travel in MMF 40, 40′. While there is some modal loss,the light 22 from SM transmitter 20 that travels through system 10 willbe limited to those modes that travel substantially down the center ofthe MMF 40, 40′.

FIG. 4 illustrates LP01 MFD of MMFs 40 with a several exemplary coredeltas versus core radii, at λ₀=1310 nm. For the purpose of the modelshown in FIG. 4 we chose the core alpha of MMFs 40 to be 2.1 but thecalculated LP01 MFDs vary very little for a range of alpha between 1.9and 2.2, over the range of core radii illustrated in FIG. 4. Forexample, we looked at the MFDs when core delta of the MMFs 40 is 1.0%.It is known that a single mode fiber SMIF-28®, produced by CorningIncorporated, of Corning N.Y. has nominal MFD of 9.2 μm for SMF-28® at1310 nm. FIG. 4 illustrates that in order for the MMF 40 to match theMFD of 9.2 μm of the SMF-28® at λ₀=1310 nm such that 0.8MFD_(SM)<LP01MFD_(MM)<1.2MFD_(SM) at λ₀=1310 nm, the core radius of the MMF 40 with1% delta should be around 10 microns (core diameter D₄₀ should be around20 microns). For example for a MMF 40 with a relative refractive coreindex delta of A=0.6%, the fiber should preferably have core diameterD₄₀ of about 15 μm, in order to have LP01 mode MFD that is approximatelyequal to the MFD of SMF-28® fiber. FIG. 4 also indicates that when thecore delta of the MMF 40 is decreased, the core radius of the MMF 40should be decreased in order for the LP01 MFD_(MM) to approximatelyequal the MFD of SMIF-28® at 1310 nm would (i.e., to enable the fiber tosatisfy the following: 0.7MFD_(SM)<LP01 MFD_(MM)<1.3MFD_(SM) at λ₀=1310nm). However, it we choose a MMF 40 with a core delta of 2.0%, the corediameter D₄₀ should be around 30 microns. Thus, FIG. 4 indicates thatwhen the core delta of the MMF 40 is increased, the core radius of theMMF 40 should be increased. FIG. 4 illustrates that for any given coredelta value of the MMF 40 we can choose a proper core diameter D₄₀ sothat the MFD of the MMF 40 is approximately (±30%) equals the MFD of thefiber 50′ (i.e., in this example MFD of SMF-28® fiber). Similar studycan be done for single mode operation around 1550 nm, or for any otherwavelength λ₀ of interest.

For example, the LP01 MFDs at λ₀=1550 nm wavelength, for different coreradii of MMF 40, at several core deltas are shown in FIG. 5. To have theLP01 MFD match (in size) the nominal MFD of 10.3 μm for SMIF-28® at 1550nm, one can choose a core diameter of 22 microns with 1% core delta or31 microns with 2% core delta. The above analysis shows that the corediameters for matching the MFDs of SMF-28® at both 1310 nm and 1550 nmare about the same for a given delta. One can choose an average diameterfor matching MFDs of both 1310 nm and 1550 nm with a very small error.For example, for 1% core delta, the core diameter can be chosen to beabout 21 μm, and for 2% delta, the core diameter can be chosen to beabout 30.5 μm.

In some embodiments of the optical system 10, for 1310 nm operation, thesingle mode fiber SMF (as a SM pigtail fiber, for example) may bedifferent from that of SMF-28® fiber, and in such case, given the MFD ofthis fiber, one can refer to FIG. 4 to find the core radius or diameterD₄₀ for a given core delta such that the MM fiber would have LP01 MFDthat is similar to that of this SMF. This same MM fiber would also workreasonably well at 1550 nm.

For example, for 1% core delta, it is determined above that 20 microncore diameter would match the LP01 of SMF-28® at this wavelength. Thesame fiber has a LP01 MFD of 9.9 micron at 1550 nm, which issubstantially similar to the 10.3 micron value for SMIF-28®. In onefurther embodiment, one can choose to use one additional mode matchingtapered single mode fiber to do mode conversion when needed.

If a smaller MMF core diameter is needed for certain applications, wecan use a matching single mode fiber (i.e., a SMF with about the sameMDF as that of the LP01 mode of the MMF) to work with it. For example,if we choose a core diameter of MMF 40 to be 30 μm for a core delta of1%, the MFD of the LP01 mode is 11.2 μm at 1310 nm, which is larger thanthat of conventional SM fiber, such as SMIF-28®. In this case we can usea single mode fiber 50 with the same or similar MFD to launch the LP01mode. As an example, a step index single mode fiber design with delta of0.25% and core radius of 5.3 μm has a MFD of 11.2 μm, which is thesubstantially the same as the MFD of LP01 mode of MMF 40.

While the exemplary MMF 40 is used for single mode or essentially singlemode transmission at a long wavelength such as either 980 nm, 1060 nm,1310 nm or 1550 nm, or any other wavelength >950 nm (or where λ₀−λ₁<100nm) where a single mode transmitter is available, the exemplary MMF 40is a multimode fiber for 850 nm VCSEL transmission, because most VCSELsto date operate around 850 nm. Preferably, the alpha value of the fibercore 42 of the MMF 40 is chosen so that the MM fiber's bandwidthperformance around 850 nm is optimal. FIG. 6 shows bandwidth vs.wavelength for several MMFs. They have the alpha s within 1.9 to 2.3range, for example of 2.096, 2.104, 2.098 and 2.092 respectively. Theresults of 50 micron core MMF with 1% are shown in FIG. 6 forcomparison. It can be shown that with smaller core and the same 1%delta, the peak bandwidth can be increased dramatically because of fewermode groups and smaller material dispersion effect. On the other hand,with 2% core delta, the maximum bandwidth is quite low. However thebandwidth the fiber with 2% core delta is still sufficient for someapplications. In some embodiments the wavelength λo is situated in 950to 1070 nm wavelength band (e.g., 980 nm or 1060 nm wavelength), or 1260nm to 1340 nm wavelength band, or 1540 nm to 1560 nm wavelength band.

FIG. 7 illustrates the refractive index profile of one exemplary MMF 40.This MMF 40 has a graded index core with alpha around 2 (i.e.,2.09<α<2.13) in order to minimize the modal group delay to achieve highbandwidth at 850 nm. The multimode fiber 40 has a modal bandwidth of atleast 2.5 GHz·Km at a wavelength λ₁ (e.g., λ₁=850 nm), preferably atleast 5 GHz·Km, and according to some embodiments at least 10 GHz·Km.Preferably, according to some embodiments, the core has a relativerefractive index delta Δ₁ (%) of at least 0.7% at 850 nm wavelength, forexample 0.7%≤Δ₁≤1.25. Table 1 shows exemplary parameters of severalembodiments of the MMF 40 (fiber Examples 1-5). All embodiments of MMF40 shown in Table 1 have MFDs in the range of 9.1 μm to 9.3 μm, which iswithin 30% of the MFD of standard single mode fibers 50′ such asSMF-28®, which has MFD of 9.2 μm at 1310 nm. The theoretical bandwidthsof the MMFs are greater than 58 GHz·km, which are much higher than thatof the standard MMFs due to fewer mode groups propagating in the MMFs40. In these exemplary embodiments 15 μm≤D₄₀≤23 μm and the multimodefiber 40 has a modal bandwidth of at least 2.5 GHz·Km at a wavelength λ₁and less than 2 GHz·Km at a wavelength λ₀≥1200 nm.

TABLE 1 MMF 40 design examples Example 1 Example 2 Example 3 Example 4Example 5 Δ₁ (%) 0.75 0.75 0.9 1.0 1.2 α 2.109 2.108 2.106 2.106 2.106r₁ (μm) 9.1 9.0 9.9 10.5 11.4 Δ₂ (%) 0 −0.4 0 0 na r₂ (μm) na 10.2 na nana D (μm) na 1.2 na na na Δ₃ (%) na −0.4 na na 0.2 r₃ (μm) na 15.0 na na22.3 W (μm) na 4.8 na na 10.9 850 nm BW 20.6 48.7 20.2 10.9 30.8 (GHz ·km) 1200 nm 1.7 2.2 2.3 1.1 1.4 BW (GHz · km) MFD @ 9.1 9.2 9.3 9.3 9.31310 nm (μm) MFD @ 10.2 10.1 10.1 10.1 10.1 1550 nm (μm)

As discussed above, according to another embodiment, a single mode fiber50 (fiber jumper 50) can be used to upgrade existing systems using 850nm standard MMF 40′ to single mode transmission at 1310 nm or 1550 nm. Astandard MMF 40′ with 1% delta has a MFD of 14.6 μm at 1310 nm, and 15.8μm at 1550 nm, and a standard MMF 40′ with 2% delta has a MFD of 13.8 μmat 1310 nm, and 15.0 μm at 1550 nm, which are much larger than the MFDsof standard SMF 50′. If a standard SMF 50′ is used as a jumper at 1310or 1550 nm, the MFD mismatch between MMF 40′ and SMF 50′ will excitehigher order optical modes, which will degrade the system's performance.This problem can be solved by using specially designed SMF 50 jumpers asshown in FIG. 8.

Some exemplary embodiments of SMFs 50 with MFDs that are similar to thatof standard MMFs 40′ are described below in Table 2, which providesparameters of SMF embodiments 50. The Example 6 fiber has a profiledesign with a depressed inner cladding surrounding the core. It has acutoff wavelength of 1288 nm. This SM fiber 50 can be used ontransmission system 10 operating at 1310 nm or a 1550 nm wavelength λ₀.If a SM fiber 50 is used only for 1550 nm, its cutoff wavelength can beincreased to improve the bending loss. In SM fiber 50 of Example 7, thecutoff wavelength is increased to 1466 nm by increasing the core delta.Example 8 SM fiber 50 has a profile design with a low index trench inthe cladding. SM fibers 50 of Examples 7-8 are designed for matching thestandard MMF with 1% core delta and 50 μm core diameter. SM fiber 50 ofExamples 9-10 are designed for matching standard MMF with 2% core deltaand 62.5 μm core diameter. Example 9 SM fiber 50 has a depressed innercladding and Example 10 has an updoped outer cladding.

TABLE 2 SMF 50 design examples Example Example 6 Example 7 Example 8Example 9 10 Δ₁ (%) 0.12 0.14 0.13 0.135 0.24 α 20 20 20 20 20 r₁ (μm)9.0 9.4 7.9 8.4 11.4 Δ₂ (%) −0.1 −0.1 −0.2 −0.12 0 r₂ (μm) 9.0 9.4 12.08.4 8.4 d (μm) 0 0 4.1 0 0 Δ₃ (%) 0 0 −0.2 0 0.2 r₃ (μm) 19 19.9 19.217.9 15.4 W (μm) 10 10.5 7.2 9.5 4 Cutoff 1288 1466 1306 1279 1301wavelength (nm) MFD @ 14.6 na 14.6 13.7 13.7 1310 nm (μm) MFD @ 15.715.8 15.7 14.7 14.8 1550 nm (μm)

Table 3 shows exemplary parameters of several embodiments of SMF 50,designed for use with a MM fiber 40′ that can operate at both 850 nmwavelength, and are capable of propagation LP01 propagation at 1060 nm.Thus, the embodiments of the fibers 50 (Example 11 and Example 12fibers) shown in Table 3 can be used in the optical transmission system10 in conjunction with such MMF.

TABLE 3 Example 11 Example 12 Δ₁ (%) 0.095 0.085 α 20 20 r₁ (μm) 8.2 7.9Δ₂ (%) −0.1 −0.1 r₂ (μm) 8.2 7.9 d (μm) 0 0 Δ₃ (%) 0 0 r₃ (μm) 17.4 16.8W (μm) 9.2 8.9 Cutoff 1030 936 wavelength (nm) MFD @980 nm na 12.6 (μm)MFD @1060 nm 13.1 13.0 (μm)

It will be apparent to those skilled in the art that variousmodifications to the preferred embodiments of the disclosure asdescribed herein can be made without departing from the spirit or scopeof the disclosure as defined in the appended claims. Thus, thedisclosure covers the modifications and variations provided they comewithin the scope of the appended claims and the equivalents thereto.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the invention. Since modifications combinations,sub-combinations and variations of the disclosed embodimentsincorporating the spirit and substance of the invention may occur topersons skilled in the art, the invention should be construed to includeeverything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. A multimode optical fiber comprising: a core witha diameter D₄₀; a cladding surrounding the core; a refractive indexprofile defined by the core and the cladding and that defines an optimalmultimode transmission at a wavelength λ₁ situated between 840 nm and860 nm, the fiber being structured to be capable of single modetransmission in an LP01 mode at an operating wavelength λo>950 nm; andthe LP01 mode having a mode field diameter LP01MFD_(MMλ0) in the range8.5 μm<LP01MFD_(MMλ0)<11 μm at the operating wavelength λo; a cutoffwavelength >1600 nm; and a modal bandwidth of at least 2.5 GHz·Km at thewavelength λ₁.
 2. The multimode optical fiber according to claim 1,wherein λo−λ₁>100 nm.
 3. The multimode optical fiber according to claim1, wherein the operating wavelength λo is in the range from 950 nm to1600 nm.
 4. The multimode optical fiber according to claim 3, whereinthe operating wavelength λo is 980 nm or 1060 nm or 1310 nm or 1550 nm.5. The multimode optical fiber according to claim 1, wherein the corediameter D₄₀ is in the range 15 μm<D₄₀<40 μm.
 6. The multimode opticalfiber according to claim 1, wherein said core has an alpha value of2.09≤α<2.2.